Lab 9. Speed Control of a D.C. motor Sensing Motor Speed - - PowerPoint PPT Presentation

Lab 9. Speed Control of a D.C. motor

Sensing Motor Speed (Tachometer Frequency Method)

Motor Speed Control Project

1. Generate PWM waveform 2. Amplify the waveform to drive the motor 3. Measure motor speed 4. Measure motor parameters 5. Control speed with a computer algorithm

microcontroller 12v dc Motor ac Tachometer Amplifier 9 v Power Supply Comparator Circuit

PWM signal pulses

Frequency Counter

Tachometer circuits

 Electrical signal carries speed information

(revolutions per unit time) in amplitude and/or frequency

 Optical encoder: disk on motor shaft alternately

blocks and passes light to a sensor

 Variable reluctance tachometer: gear teeth pass a

magnetic pickup

 Pickup coil/generator: voltage induced on

separate winding in the motor

Pickup coil (Buehler motor)

 Voltage induced in separate coil at one end of rotor  Both frequency and amplitude of the generated signal

are proportional to motor speed

vtach(t) = Kωsin(ωt)

ω = rotational speed K is a constant (depends on windings and geometry) DC offset = 0v

Tachometer output

<= PWM signal applied to motor

(100 Hz. 50% duty cycle)

<= Tachometer output

(118.3 Hz, 2.68v p-p)

Frequency measurement methods

1.

Convert frequency to an analog voltage, and then to digital form



frequency-to-voltage converter IC



digitize voltage level with A/D converter

2.

Count # signal periods per unit of time



frequency = # periods / time



count periods with programmable timer/counter



useful for higher frequencies

3.

Measure one signal period (T)



frequency = 1 / T



measure period with programmable timer



useful for lower frequencies

 Convert tachometer output to a digital waveform

 Tachometer output signal: sinusoid with 0 V dc offset  Amplitude ranges from 0 V to well over 12 V peak

(Measure in lab for min and max speeds)

 Desired form: square wave, oscillating between 0 and 3 V

 Convert with an analog “comparator”

 Vout = 0 V (logic 0) for V1 < V2  Vout = 3 V (logic 1) for V1 > V2

Methods 2 & 3: signal conditioning

Vout V1 V2 +

LM111/LM211/LM311 voltage comparator

 Nearly identical, except for temperature range

 LM111 [-55oC…+125oC] (military grade)  LM211 [-25oC…+85oC] (industrial grade)  LM311 [0oC…+70oC] (commercial grade)

 Power supply range = ±5 V to ±15 V  Input voltage range = ±30 V  Output drives loads between ground and positive

supply value

 Pull-up resistor needed from output to positive supply

 Output balancing and strobe capability

LM111 / LM211 / LM311 Package

Pin# Function (lab values) 1. Ground (0 V) 2. V1 input 3. V2 input 4.

• V supply (-9 V)

5. Balance** 6. Balance/strobe** 7. Vout (open collector) (pull-up resistor to +3v) 8. +V supply (+9 V) **short pins 5-6 together

GROUND 1 INPUT 2 INPUT 3

• V 4

8 +V 7 OUTPUT 6 BALANCE/ STROBE 5 BALANCE Dual-In-Line (DIP) Package Top View

+ _

Comparator signal & reference voltages

(V1 and V2)

 Goal: V1 > V2 approximately half of each period, to

produce square wave at Vout

 Option 1

• V1 = ac signal
• V2 = dc offset of the ac signal



V2 = signal with sinusoid removed by a low pass filter



OR, apply a constant voltage to V2 ≈ dc offset

 Option 2

• V1 = ac signal with dc offset removed by high pass filter
• V2 = ground (0v)

Buehler motor tachometer signal offset ≈ 0v. Which option would be more efficient?

Design & verify comparator circuit

 Model in PSPICE or Multisim  LM311 comparator (or LM211 or LM111), resistors,

DC voltages, etc. found in libraries

 Use a VSTIM (voltage stimulus) generator to model

the optical encoder

 Simulate to verify square wave output over the

range of optical encoder signal frequencies and amplitudes, corresponding to “useful” motor speeds

 Use voltage probes to examine signals  Measure expected frequencies in lab for min/max speeds

 Implement circuit and compare actual operation to

simulation of the modeled circuit

Example model

VSTIM from library “sourcstim” R,C from “analog” lib. LM111 from “eval” lib. VDC from “source” lib. AGND from “port” lib. 5v was used here. 3v should be used.

Voltage divider to reduce amplitude

Discovery Board

• Internal pull-up

to 3v on GPIO?

• 3v output pin?

Simulation

ac signal (blue) volt-divider

• utput

(red) comparator

• utput

(purple)

Simulation – undesirable results

Ground instead of negative supply on pin -V Input voltage range exceeds +V/-V supplies

Signal conditioning review

 Convert ac signal to digital signal  Measure period with timer  Design challenges:

 ac signal exceeds comparator voltage ratings

 reduce with voltage divider?

 ac signal may be noisy

 may cause "false" transitions  introduce hysteresis or filter?

T

STM32 timer “input capture” mode

16

TIMx_CCRy latches TIMx_CNT value when transition detected on input TIMx_CHy

• CCxIF flag sets, and interrupt generated if enabled (CCxIE=1)
• Detected signal edge is programmable (rising, falling, both)

Example: Use two channels to measure PWM duty & period via opposite edges Reset CNT=0

CCR2=CNT=2 CCR1=CNT=4 (duty) (period) Falling edge Rising edge

General-purpose timers TIM10/TIM11

17

16 MHz

Capture/Compare Channel 1 Input/Output = TIMx_CH1

(other timers have additional channels) Basic timing function (earlier labs) Timer input Input filter & Prescaler Capture/Compare edge detector Register

Input capture mode

 Input pin: TIMx_CHy (ex. TIM11_CH1, accessible at pin PA7)



Connect a GPIO pin to timer input TIMx_CHy



Select alternate function mode for the pin in MODER



Select TIMx_CHy as the alt. function input in TIMx->AFR[0] Example: Pin PA7 => TIM11_CH1 Pin PA6 => TIM10_CH1



TIMx_CCRy = TIMx capture/compare register, channel y



Use TIM11->CCR1 (only one channel in TIM10 and TIM11)



Could also use TIM10, but it is generating the PWM signal to drive the motor.



TIMx_CNT value captured in TIMx_CCRy at time of event on input TIMx_CHy



Captures time (count) at which the event occurred



Use to measure time between events, tachometer signal periods, etc.



TIMx_CNT operates as discussed previously



Trigger update event and reset to 0 when CNT = ARR (up-counter)



For best results:



Reset TIMx->CNT to 0 after each capture event (captured CNT = desired period)



Set TIMx->ARR to a value greater than expected period (prevent update event)

18

Configure the GPIO alternate function

 Refer to User Manual to determine which GPIO pin is

able to connect to TIMx_CHy Example: TIM11_CH1 connects to PA7

 In MODER, configure the GPIO pin as AF mode  In the GPIO AF register, select TIMx_Chy  Configure GPIO PUPDR register if pull-up or pull-down

desired**

 This should match the edge detection setting (rise or fall)  For example, use pull-up if detecting rising edge

** Recall that the LM311 comparator requires a pull-up resistor between its output and +3 V.

Timer configuration

Basic timer setup same as previously discussed

 TIMx_CNT: 16-bit counter 

Set to 0 at start of period, so captured value = period

 TIMx_ARR: auto-reload value 

Set to value > max period to prevent update event before capture

 TIMx_PSC: prescale value 

Prescale the clock, if necessary, to measure larger periods

 TIMx_CR1: control register 1 

CEN=1 to enable counter

 TIMx_SR: status register ; TIMx_DIEN: interrupt enables 

CC1IF sets on capture event for channel 1



Interrupt when CC1IF sets, if CC1IE=1



UIF sets on update event (TIMx_CNT overflow), interrupt if UIE=1

Input stage includes digital filter, edge detection, multiplexing and prescaler



Filter: sample input signal after an event to ensure it’s not “noise”



Edge detector: detect rising edge, falling edge, or both



Divider/prescale: capture every event (typical), or every 2nd, 4th or 8th event



Configure in Capture/Compare Mode Register (CCMRx) and Capture/Compare Enable Register (CCER)

Capture/Compare Channel Inputs

Which edge(s)? Filter options From GPIO input pin To capture register Input-select Prescale Enable

Capture/compare mode register 1

(Input capture mode)

Capture/Compare 1 Select

00 = output 01 = input: IC1 = TI1 10 = input: IC1 = TI2 11 = input: IC1 = TRC

Input Capture 1 Filter

Sampling frequency for TI1 input, plus Length of digital filter applied to TI1 (see next slide)

Input Capture 1 Prescaler

00: capture on every event 01: capture on every 2nd event 10: capture on every 4th event 11: capture on every 8th event

CC1S = Input mode =>

• Bits 15-8 configure

Channel 2 (same order)

• CCMR2 configures

Channels 3/4

TIMx_CCMR1 (reset value = all 0’s)

Suggestion: First try default IC1F/IC1PSC settings

Input Capture Filter

 IC1F (Input Capture 1 Filter) selects sampling frequency

and #samples (N) needed to validate a transition on the input.



Example: If IC1F = 0001, and set to capture rising edge,



When rising edge detected, sample the channel twice with FCK_INT.



If both samples are high then the capture is validated. Otherwise, no event. IC1F IC1F fDTS = Dead Time and Sampling clock = 1/2/4 fCK_INT (select in TIMx->CR1)

Capture/compare enable register (Input capture mode)

24

TIMx_CCER (reset value = all 0’s)

CC4: bits 15-12 CC3: bits 11-8 CC2: bits 7-4 (same order as CC1)

CC1 Polarity: CC1NP/CC1P select capture trigger: 00: rising edge of input 01: falling edge of input 11: both edges of input CC1 Enable: 1 = Capture enabled 0 = Capture disabled Must enable capture and select capture trigger

Example: Wind Speed Indicator (Anemometer)



Rotational speed (and pulse frequency) proportional to wind velocity



Two measurement options:



Frequency (best for high speeds)



Width (best for low speeds)



Can solve for wind velocity v



How can we use the Timer for this?



Use Input Capture Mode to measure period T of input signal T1 T2 T Anem_Out

Input Capture Mode for Anemometer

 Operation (repeat continuously):

 First capture - on rising edge (Crising_1)  Clear counter, start new counting  Second Capture - on rising edge (Crising_2)  Read capture value, save for wind speed calculation  Clear counter, start new counting

 Solve the wind speed

Vwind = K÷(Crising_2 – Crising_1)×Freq_cnt

 Or, if count reset to 0 on each rising edge:

Vwind = K÷(Crising_2)×Freq_cnt

Set up for Anemometer measurement



Apply Anem_Out signal to pin PD15



TIM4_CH4 is an alternate function for PD15 (from data sheet)



Configure PD15 as alternate function in GPIOD->MODER



Select alternate function TIM4_CH4 for PD15 in GPIOD->AFRH



Configure TIM4_PSC and TIM4_ARR for TIM4 counting period



Best if counting period > time to be measured (avoid overflow interrupt)



Reset TIM4_CNT to 0 after each capture



TIM4_CCMR2 Capture/Compare mode register 2 (Channel 4)



Set CC4S to map IC4 to TI4



Set IC4F, IC4PSC to defaults (no filter or prescale)



TIM4_CCER Capture/compare enable register



Set CC4E to select “input” mode



Set CC4N:CC4P = 00 to select rising-edge (01 for falling edge)



TIMx_DIER DMA/interrupt enable register



Set CC4IE to enable interrupt on input capture event (to read captured value)



TIM4_CR1 Control register: Set CEN to enable the counter



TIM4_SR Status register: CC1IF indicates input event occurred (clear by software)



TIM4_CCR4 Capture/Compare register: captured value of TIM4_CNT



TIM4 Interrupt handler:



Read TIM4_CCR4 to get period, reset TIM4_CNT, reset CC1IF, calculate wind speed.

Lab Procedure

 Simulate comparator circuit in PSPICE to verify circuit & values

 Verify that a square wave (0 to 3 V) is produced

 Re-verify motor speed controller from Lab 8



Components can be damaged with incorrect connections/operation!



Triple-check power/ground connections!

 Incorporate comparator into your circuit  Verify comparator inputs & square wave output on o’scope  Modify software to measure square wave period  Measure ac tachometer signal period* for each of the 11 keypad-

selected settings (11th setting is stopped)

 Plot:  Signal period* vs. measured motor speed  Signal period* vs. PWM signal duty cycle

*Measured by the µC via input capture